Washington, USA — California wildfires pumped nearly 8 million metric tonnesof climate-warming carbon dioxide into the atmosphere in just a week, aboutone-quarter as much as fossil fuels do in that state in a month, scientists saidon Wednesday.

The release of carbon dioxide in wildfires is part of the natural cycle in whichburning plays an important role, the scientists reported in the online journalCarbon Balance and Management. And the ebb and flow of carbon that isalternately sucked up and emitted by plants is different from that spewed byfossil-fueled factories and vehicles.

Overall, the study estimates that fires in the contiguous United States andAlaska release about 290 million metric tons of carbon dioxide a year,equivalent to 4 to 6 percent of U.S. emissions from the burning of fossil fuelslike coal and oil.

These figures are hard to pin down because of the challenge of figuring outhow much carbon was tied up in the plants that burned, and what percentage ofthe plants actually went up in flames in different kinds of fires, said JasonNeff of the University of Colorado at Boulder, a co-author of the study.

The estimates carry a 50 percent margin of error, but that still means U.S.wildfires emission are equivalent to between 2 and 8 percent of emissions fromfossil fuel burning, which Neff noted is a considerable fraction.

Big fires like the ones that destroyed more than 2,000 homes in Californiathis month can have a particularly heavy carbon footprint, the researchers wrote.

“A striking implication of very large wildfires is that a severe fireseason lasting only one or two months can release as much carbon as the annualemissions from the entire transportation or energy sector of an individual state,”the authors write.

Because planetary climate change is expected to bring more frequent andwidespread wildfires, these could release more carbon dioxide into theatmosphere as the planet warms, in what scientists call a positive feedback loop– a cycle that feeds on itself, spurring global warming.

Neff and Christine Wiedinmyer of the National Center for Atmospheric Researchdrew a distinction between carbon dioxide released by periodic wildfires andthat released by fossil fuel burning.

Carbon emitted by fires will eventually be absorbed by plants as forests andgrasslands regenerate after the blaze. But modern trends to suppress fires canhave an unintended effect of giving the next fire more plant life to feed upon,since forests have the time to grow more densely.

Carbon emitted by fossil fuel burning is carbon that was locked undergroundfor hundreds of millions of years, and after it is released, it will not bereabsorbed by the ecosystem on any human timetable. This is a large release ofclimate-warming gas, Neff said.